The present invention relates to an apparatus and a method for detecting surface flaws or contaminants of transparent members, and more particularly, to an apparatus and an automated method using the apparatus for inspecting and detecting, using electromagnetic radiation, the presence of surface flaws and contaminants in the planar or curved surfaces of members which are transparent to the electromagnetic radiation.
Precision components in a great variety of applications require for their proper operation that their surface be essentially free of flaws and contaminants. A significant fraction of such components are made of materials which are transparent to electromagnetic radiation. Examples are the glass substrata used in Liquid Crystal Displays (LCD's) or other Flat Panel Displays (FPD's), windows for Charge Coupled Devices (CCD's) detectors, contact lenses, miniature lenses, glass substrata for rigid disks, etc. For these and similar components, it is imperative that the critical surfaces be virtually free of surface flaws.
In addition, the present invention can also be applied in the inspection of low contrast marks engraved or coated on transparent elements.
Surface flaws can be of a variety of types. The word `flaw` as used herein is meant to include all types of surface defects such as scratches, digs, pits, chips, and other abnormalities and irregularities which tend to create surface discontinuities. Included also are contaminants which adhere to, or are in contact with, the outer surface of the transparent member, including also contaminants and defects in transparent coatings on the surface.
Since surface flaws are typically very small, it is normally very difficult to detect the defects, specially with high speed automatic systems.
Surface flaw inspection on transparent objects is currently performed largely by visual inspection carried out by human inspectors. The process is labor-intensive and often unreliable.
The inspection process has been automated to some extent. There are currently a number of instruments which can aid in the inspection for surface flaws. For example, systems are known which illuminate the member inspected with a flying spot and which detect the scattered light in the black zone. It is also known to use interferometry principles by detecting interference-patterns created by the interaction of light which passes through the inspected element and a reference light beam.
However these methods suffer from low detectability capabilities, a slow inspection rate which results in low inspection rates, and difficulty in the inspection on non-flat surfaces.
In addition, the existing methods cannot be accurately applied for the inspection of defects in low contrast marks engraved or coated on transparent elements such as in reticles, conductive lines, etc.
There is thus a widely recognized need for an apparatus and a method of inspecting the surface of transparent elements for flaws which can accurately detect the presence of such flaws and which can be used in an automated fashion requiring little or no human judgment.
It would be desirable to have an inspection system for detecting surface flaws which would quickly and reliably determine the presence of surface flaws.